Strategies for sustainable morphological remediation of the Tamar river estuary and other similarly degraded estuaries

When anthropogenically induced morphological change within estuaries becomes irreversible, hysteresis is established and mitigating projects are required to undo any degradation. The morphologically degraded Tamar River estuary in Tasmania Australia typifies the universality of problems and solutions pertaining to estuaries in Australia, North America, Europe and the UK. Historically, restoration projects proposed for the Tamar were analogues of projects elsewhere, for which full repercussions are emerging. Barrages and weirs for example, unintentionally altered the hydrodynamics with negative long-term morphological consequences. Within the Tamar, recently mooted projects supposedly reduced the flood dominance of tides. Since the 1880s, silt management was by dredging (now cost prohibitive); replaced in recent years by silt raking. Neither process addresses the root-cause of the degradation. The aim of this thesis was to develop and model sustainable solutions (proven and novel) for hysteretic degraded estuaries such as the Tamar, whilst identifying projects having potentially unacceptable trade-offs. The multifaceted methodology reviewed datasets pertaining to tidal heights, bathymetry and land elevations for the Tamar; examined asymmetrical tides, net sediment flux and establishment of equilibrium; developed a First Order Morphological Response Model (FORM) to review and evaluate proposed projects; used FORM on four international examples, and evaluated the effectiveness of barrages as mitigators of sea-level rise. Tidal asymmetry results showed that flood velocity bias persisted and maximised at equilibrium, the inference being their removal is counterproductive. A feedback loop developed at equilibrium (driven by M4 and M6 harmonics); deposition on the banks led to slumping into the constantly scoured channel resulting in a net zero sediment flux. FORM modelled the new equilibrium of five putative barrages within the Tamar. Equilibrium established from a persistent flood velocity bias, ultimately resulting in a net loss of physical amenity below the barrage and unsatisfactory ecological trade-offs both upstream and downstream. Another study of four international barraged systems showed morphological degradation with some mitigation of sea-level rise in each head-pond but potential to increase tidal ranges downstream, thereby exacerbating sea-level rise. Neither study justified the installation of a barrage in the Tamar. Projects designed to remove or mitigate reduced estuarine and freshwater flow would be beneficial in restoring some amenity to the Tamar with potentially up to 8 million m3 of silt permanently removed. Those projects synergistic with nature proved prospective, whereas those antagonistic to nature would fail. Results inform a more sustainable management of estuaries in general with emphasis on a proactive, rather than a reactive paradigm.